Keep your eye on the ball

For all purposes, our nervous system has one hell of a difficult task to accomplish – it needs to take in numerous stimuli all occurring either simultaneously or in some sort of temporal order and determine if there is a pattern worth responding to. While other tissues only need to make a few hormones or make some polymer twitch, neurons need to be constantly opening and closing protein gates to balance various ions. In other words, your brain does a lot of work, and therefore uses the lion’s share of energy your body takes in to do so.

Change blindness is one of those classic demonstrations of the gaps that exist in our perception where our nervous system has evolved to take a bit of a gamble. While the specific neurology of it is still questioned, it is an indication that sensing visual information isn’t the same as observing it, and our short term memory isn’t always as reliable as we might believe. A flicker, or momentary change of attention, is all it takes to reset our comprehension of what sits in front of us in plain sight. Take this aircraft, for example. It might take you a while to notice what is missing*.

Psychologists from Harvard University have demonstrated a rather interesting illusion which goes further in showing how easily overwhelmed our perception can be through simple change and movement. There findings are due to be published in Current Biology.

Watch the clip below by paying close attention to the cross in the middle. The dots will change colour for about five seconds before the surrounding ring begins to rotate back and forth.

Now, watch it again, only this time watch one of the coloured dots. Where before they appeared to cease changing, on closer observation they do nothing of the sort.


The phenomenon has been termed ‘silencing’, and describes how it’s difficult to recognise change in a moving object. There could be two reasons this happens. One is that the observer sees the original state of an object and simply doesn’t update it with the new information (termed ‘freezing’). The other possibility is that the observer sees whatever the current state is, but doesn’t mark a change between the two therefore isn’t aware of any moment of difference (known as ‘implicit updating’).

Both explanations are shown to occur with other forms of illusion. But the results of the study indicated that the illusion is a case of seeing different colours without seeing them change.

The likely reason for this has to do with how our retina maps onto our brain. As a moving image drifts across the back of our eye, it proceeds to activate a line of photoreceptors, which in turn send those messages to a corresponding line of tissue in our brain’s visual processing system. Normally, a static, changing image will prompt a nervous reaction that says ‘this pattern isn’t like it was before’. But, if it moves quickly enough from one patch of neurons to another, the same resources aren’t capable of performing this act of recognition.  To do so would demand something a little more complicated than we currently have.

If the eye tracks the changing object, however, the object remains on the same section of retina, allowing it to persist long enough in the corresponding section of the visual processing system for a variation to become apparent. The take-home-message from this is that if your eye isn’t tracking it, change simply isn’t important enough for the brain to waste energy on bringing it to your awareness.

Yet another example of how our circuitry isn’t wired for truth, but rather for economy.

*Check out the engine beneath the wing.

For additional illusions used in the study, see this Harvard website.

Published in: on January 9, 2011 at 10:37 pm  Leave a Comment  
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The persistence of illusion

The thing I love most about optical illusions is their persistence in the face of knowledge. No matter how much you know about an illusion, about the overall context, the neurology behind it, who created it or how they did it, it’s impossible to shake that sense of dissonance.

As an activity for Science by Email this week, I simply went out and photographed a building in Canberra with a subtle tilt of the camera, and then duplicated it and presented the images side by side.

Building illusion

Which looks as if it is leaning the most? Of course, they’re the same image, but it probably won’t stop a lot of you from questioning that.

If you happen to be in Canberra on Saturday the 14 August, I’ll be presenting on optical illusions in art (mostly using cartooning) at 1.30 at the Shine Dome for the Australian Science Festival. Come along and say hi.

Published in: on July 23, 2010 at 8:52 am  Comments (1)